In the manufacture of semiconductor devices, photolithographic techniques are often used. Generally, this involves projecting the image of a reticle onto a photosensitive resist covered wafer and subsequent processing to create a semiconductor device. While there are many different types of photolithographic systems used in the manufacture of semiconductor devices, one type of device or tool providing imaging of very small linewidths or featured sizes is a scanning photolithographic system. One such system is sold under the trademark MICRASCAN by SVG Lithography Systems, Inc., Wilton, Conn. In a scanning photolithographic system, a rectangular shaped illumination field or slit is scanned at a predetermined rate across a reticle being imaged onto a photosensitive substrate, such as a photosensitive resist or photoresist covered wafer. The reticle and photosensitive substrate move synchronously with each other at different rates to accommodate any magnification or reduction of the image of the reticle by the projection optics used to project the image of the reticle onto the photosensitive substrate. As the feature sizes or linewidths of the various elements or circuit patterns being reproduced onto the photosensitive substrate decrease in size, there is a need to increase system performance. While projection optics have advanced considerably and are a major influence on image quality, the illumination used to project the image of the reticle onto the photosensitive substrate is also critical to system performance and quality of the finished semiconductor device. While there have been many attempts to provide more improved illumination with various illumination sources, most of these efforts have been directed to providing a uniform illumination. A device for modifying illumination used in a photolithographic device is disclosed in U.S. Pat. No. 4,516,852 entitled "Method and Apparatus for Measuring Intensity Variations in a Light Source", issuing to Liu et al on May 14, 1985, which is herein incorporated by reference. Therein disclosed is an arcuate slit that is adjustable with a deformable band. Another system for improving illumination used in a scanning photolithographic system is disclosed in U.S. patent application Ser. No. 09/023,407 filed Feb. 12, 1998 and entitled "Adjustable Slit and Method for Varying Linewidth", which is herein incorporated by reference. While these prior devices are useful in adjusting the illumination properties of the illumination slit used to scan a reticle, they have generally only been beneficial in improving image quality along a single axes perpendicular to the direction of scan. Therefore, there is a need to further improve system performance and image quality in a direction along the direction of scan.